📌 fix: Pin the PTC Trace Through Its Final Settle

The fourth Codex pass on `4bf68e1`, one P2 finding.

`useFollowScroll` returned early whenever `active` was false, so the one
change it most needed to follow was the one it skipped. A failing inner call
settles by appending its error line in the same commit that clears the last
running row: the content grows and the stream ends together, and the pin that
would have revealed that line never fired. On an expanded, bottom-pinned pane
the failure — the row a reader most wants — stayed below the fold.

The falling edge of `active` now pins too, but only when the content changed
with it. Ending a stream on its own still leaves the pane where the reader
left it, which is what the existing contract promises and what the sibling
code and command panes rely on; a reader who has scrolled up is untouched
either way.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MBRQUsPXSvDBnjrSDoXWHP

client/src/components/Chat/Messages/Content/Parts/__tests__/useFollowScroll.spec.tsx

@@ -112,6 +112,34 @@ describe('useFollowScroll', () => {
     expect(state.writes).toEqual([900]);
   });
 
+  /** A failing PTC call settles by appending its error line in the same
+   *  commit that clears the last running row, so the growth and the end of
+   *  the stream arrive together. */
+  it('pins one last time when the change that ends the stream also adds content', () => {
+    const { rerender, state } = setup();
+    state.scrollHeight = 1200;
+    rerender(<Probe content="ab" active={false} expanded />);
+    expect(state.writes).toEqual([1200]);
+  });
+
+  it('does not pin the settling change for a reader who scrolled up', () => {
+    const { rerender, pane, state } = setup();
+    state.scrollTop = 100;
+    fireEvent.scroll(pane);
+    state.scrollHeight = 1200;
+    rerender(<Probe content="ab" active={false} expanded />);
+    expect(state.writes).toHaveLength(0);
+  });
+
+  it('pins only the settling change, not the ones after it', () => {
+    const { rerender, state } = setup();
+    rerender(<Probe content="ab" active={false} expanded />);
+    expect(state.writes).toEqual([900]);
+    state.scrollHeight = 1200;
+    rerender(<Probe content="abc" active={false} expanded />);
+    expect(state.writes).toEqual([900]);
+  });
+
   it('detaches when the user scrolls up beyond the follow threshold', () => {
     const { rerender, pane, state } = setup();
     state.scrollTop = 100;

client/src/components/Chat/Messages/Content/Parts/useFollowScroll.ts

@@ -32,8 +32,16 @@ const FOLLOW_THRESHOLD_PX = 40;
  * would open at the tail. Gated, it opens reading from the top, while an
  * expand mid-stream pins immediately. The pin's own scroll event measures
  * distance zero and keeps the attached state, so programmatic scrolls
- * need no special-casing. Finished panes (`active === false`) are never
+ * need no special-casing.
- * scrolled.
+ *
+ * A finished pane (`active === false`) is never scrolled, with one
+ * exception: the very change that ends the stream. That change often adds
+ * content — a failing PTC call settles by appending its error line in the
+ * same commit that clears the last running row — and gating on `active`
+ * alone would drop exactly the pin that content needs, leaving the final
+ * line below the fold. So the falling edge of `active` still pins, but
+ * only when the content changed with it; ending the stream on its own
+ * leaves the pane where the reader left it.
  */
 export default function useFollowScroll<T extends HTMLElement>(
   content: string | readonly ReactNode[],
@@ -42,6 +50,8 @@ export default function useFollowScroll<T extends HTMLElement>(
 ): { ref: RefObject<T>; onScroll: UIEventHandler<T> } {
   const ref = useRef<T>(null);
   const followRef = useRef(true);
+  const wasActiveRef = useRef(active);
+  const previousContentRef = useRef(content);
 
   const onScroll = useCallback<UIEventHandler<T>>((event) => {
     const el = event.currentTarget;
@@ -49,7 +59,14 @@ export default function useFollowScroll<T extends HTMLElement>(
   }, []);
 
   useLayoutEffect(() => {
-    if (!active || !expanded || !followRef.current) {
+    const settling = wasActiveRef.current && !Object.is(previousContentRef.current, content);
+    previousContentRef.current = content;
+    wasActiveRef.current = active;
+
+    if (!expanded || !followRef.current) {
+      return;
+    }
+    if (!active && !settling) {
       return;
     }
     const el = ref.current;